This invention relates to a thin-film semiconductor device for driving active matrix type displays, image sensors, printer heads, etc. More particularly, this invention relates to an improved thin-film semiconductor device that can operate at higher speed.
Thin-film semiconductor devices of the type contemplated by the present invention are described hereinafter with particular reference to MOS thin-film transistors. Two major types of thin-film MOS transistors are a "reverse staggered" transistor and a "staggered" transistor. A "reverse staggered+ thin-film MOS transistor is shown in FIGS. 10 and 11, and its principal part is typically composed of the following components: a glass substrate g; a gate electrode gt formed on the glass substrate g; a gate insulating film h that covers the gate electrode gt; a first semiconductor layer j that is made of amorphous silicon and that is superposed on the gate insulating film h; a protective film k formed on the first semiconductor layer j; and a source electrode st and a drain electrode dt that are formed at opposite ends of the first semiconductor layer j via a second semiconductor layer m made of n.sup.+ -amorphous silicon. A "staggered" thin-film MOS transistor is shown in FIGS. 12 and 13, and its principal part is typically composed of the following components: a glass substrate g; a first semiconductor layer j that is made of amorphous silicon and that is formed on the glass substrate g; a source electrode st and a drain electrode dt that are formed at opposite ends of the first semiconductor layer j via a second semiconductor layer m made of n.sup.+ -amorphous silicon; signal wirings n respectively connected to the source electrode st and the drain electrode dt; a gate insulating film h that covers the first semiconductor layer j; and a gate electrode gt formed on the gate insulating film h.
In order to operate the thin-film MOS transistor of the either type, a drain voltage V.sub.D is applied between the source electrode st and the drain electrode dt and, at the same time, a gate voltage V.sub.G is applied to the gate electrode gt, whereupon a channel is formed in the first semiconductor layer j and the transistor is turned on to produce a flow of drain current I.sub.D. As the gate voltage V.sub.G is lowered, the channel in the first semiconductor layer j diminishes until the transistor is turned off to cease the flow of drain current I.sub.D. Being operated in this way, thin-film MOS transistors are used to drive active matrix displays, image sensors, printer heads, etc.
In the fabrication of thin-film semiconductors of the type contemplated by the present invention, the "first wiring members" such as the gate electrode gt of the reverse staggered type, and the source electrode st, drain electrode dt and signal wirings n of the staggered type, that are to be formed in the first stage, must be made of a conductive material that has good adhesion to the glass substrate g (insulating substrate) and that is sufficiently heat-resistant to avoid deterioration in subsequent heat treatments. To meet these requirements, high-melting-point (refractory) metals such as tantalum (Ta), molybdenum (Mo), titanium (Ti) and chromium (Cr) have conventionally been used and, among these metals, tantalum is used most extensively since it has high resistance to galvanic corrosion and contributes to a higher withstand voltage due to the formation of an anodic oxidation film.
A problem with the use of tantalum is that when a thin Ta film is formed by the sputtering on an insulting substrate such as a glass substrate, it will assume the .beta.-tantalum form having a tetragonal lattice structure. Since .beta.-tantalum has relatively high electric resistance, considerable delay in signal propagation occurs in semiconductor devices that use tantalum and this has been a major obstacle to the previous attempts to increase the operating speed of those devices.
As an alternative to .beta.-tantalum, tantalum alloys such as TaW and TaMo that have better conductivity than .beta.-tantalum are currently used in several areas of the semiconductor industry. However, when such tantalum alloys are applied to form the gate electrode and other first wiring members, the resulting anodic oxidation film will not contribute to a higher withstand voltage. Further, such tantalum alloys are less conductive than .alpha.-tantalum.